Material steels for bolts having a tensile strength less than 1100 MPa are now replaced from standardized steels to boron-added steels so as to have lower cost. However, SCM steels (chromium molybdenum steels) and other standardized steels are still heavily used for bolts having a higher tensile strength of 1100 MPa or more. The SCM steels contain large amounts of alloy elements such as Cr and Mo. Demands are increasingly made to provide SCM-alternate steels containing lower amounts of Cr and Mo so as to reduce the steel cost. Simple reduction of alloy elements, however, may hardly help steels to offer a strength and delayed fracture resistance both at satisfactory levels.
Under such circumstances, boron-added steels have been considered as materials for high strength bolts, because the boron-added steels effectively offer better hardenability by the addition of boron. The boron-added steels, however, offer significantly inferior delayed fracture resistance with an increasing strength, and it is difficult to apply them to a portion in a severe use environment.
A variety of technologies for the improvement of delayed fracture resistance has been proposed. Typically, Patent literature (PTL) 1 proposes a steel having better delayed fracture resistance by specifying the contents of elements such as V, N, and Si. Simple specification in the element contents, however, difficulty help the steel to have a strength, delayed fracture resistance, and corrosion resistance all at satisfactory levels.
PTL 2 proposes a bainitic steel having small unevenness in mechanical properties. The bainitic steel, however, is hardly applicable to a bolt because the bainitic phase causes the steel to have inferior wire drawability and cold forgeability.
PTL 3 proposes a case-hardening boron-added steel having little heat treatment strain. The case-hardening boron-added steel, however, is hardly applicable to a bolt because the steel, when undergoing carburizing and quenching, has a higher hardness in its surface layer and offers significantly inferior delayed fracture resistance.
PTL 4 and PTL 5 propose technologies for refining grains so as to offer better delayed fracture resistance. The steels, however, are hardly applicable to a severer environment when the steels enjoy the effects of grain refinement alone.
All the technologies previously proposed for better delayed fracture resistance are disadvantageous in at least one of strength, delayed fracture resistance in a severe environment, and manufacturing.